STUDIES OF THE CALCIUM BINDING SITES OF PORCINE AND BOVINE TRYPSIN USING LANTHANIDES AS PROBES.

Lanthanides, and in particular Tb**3** plus , can be used to probe the metal-ion binding site of the procine enzyme. Binding of Tb**3** plus to procine trypsin results in an enhancement of its fluorescence. The fluorescence enhancement results from energy transfer from one or more tryptophans in close proximity to the bound metal ion. The studies have been extended to the bovine enzyme. It is found that binding of Tb**3** plus to this protein also results in an enhancement of the fluorescence. The observed enhancements are summarized. It is seen that for all the excitation frequencies and the corresponding emission peaks the enhancement observed with the bovine enzyme is about an order of magnitude weaker than that found with porcine trypsin. It was found, in addition, that the structure of the terbium emission spectra is different for the two enzymes. These results suggest that the ligand configuration around the bound cation is different in the two proteins.